1. Field of the Invention
The present invention relates to systems and methods receiving broadcast signals, and in particular to a system and method for receiving simultaneous terrestrial and satellite broadcasts sharing the same channel using an antenna with posterior sidelobes and anterior nulls.
2. Description of the Related Art
It has been proposed to cooperatively share the current Broadcast Satellite Service (BSS) frequency bands to allow additional programming material to be transmitted to BSS users or subscribers using the same frequency bands as currently used by BSS satellites. This is implemented through the use of terrestrial based transmitters to transmit the additional programming.
One such system is disclosed in U.S. Pat. No. 5,761,605, issued to Tawil et al. on Jun. 2, 1998, which patent is hereby incorporated by reference herein. This patent describes a system in which transmissions from the terrestrial transmitter are received by a second antenna at the user""s premises that faces in the opposite azimuthal direction from the BSS satellite receive antenna.
Fundamental to any such system, including the system disclosed in the ""605 patent, is that the isolation between the signals from the terrestrial source and the satellite source must be sufficient to prevent interference. While the foregoing system applies to simultaneous channel sharing between satellite and terrestrial transmitters, it requires a BSS satellite ground antenna having highly directional, monocular sensitivity characteristics in order to realize low interference levels. Such antennae can be difficult and expensive to design and produce. Further, it is impractical to replace or modify the several million BSS satellite receive antennae now in use which do not have the desired highly-directional monocular sensitivity. Thus, any simultaneous use of the BSS bandwidth for terrestrial transmissions according to the ""605 patent would cause large areas of unacceptably high interference with existing systems.
What is needed is a system and method that provides significantly improved interference isolation to provide simultaneous channel sharing without requiring a satellite ground antenna (e.g. a BSS antenna) with highly directional sensitivity characteristics. Preferably, such a system and method would permit terrestrial re-use of the subject BSS broadcast frequencies, with reduced and more acceptable potential interference levels, particularly when used with standard BSS antennae already in widespread use. The present invention satisfies that need.
To address the requirements described above, the present invention discloses a method and apparatus for simultaneously receiving a first signal from a non-terrestrial source and a second signal from a terrestrial source on the same or overlapping channels.
In one embodiment, the apparatus comprises at least one terrestrial transmitter transmitting information on at least one frequency simultaneously usable by at least one satellite transmitting to a satellite receive antenna having a sensitivity characterizable by a primary sensitive axis directed substantially at the satellite. The terrestrial transmitter includes an azimuthal gain characteristic directed substantially away from the Earth""s Equator. In another embodiment, the terrestrial transmitter is disposed at a location defining a vector angularly displaced from the primary sensitive axis by an angle of less than 90 degrees.
In still another embodiment, the apparatus comprises a terrestrial receive antenna for receiving a first signal from a terrestrial transmitter on a frequency usable by at least one satellite transmitting to a satellite receive antenna having a sensitivity characterizable by a primary sensitive axis directed substantially at the satellite. The terrestrial receive antenna includes a primary sensitive axis directed substantially in the direction of the Earth""s Equator. In another embodiment, the terrestrial receive antenna sensitive axis is displaced away from the satellite receive antenna primary sensitive axis by an offset angle of less than 90 degrees.
In yet another embodiment, the present invention includes a first antenna having a sensitivity characteristic with a plurality of lobes and nulls therebetween and a second antenna having a sensitivity characteristic with a plurality of lobes and nulls therebetween. The first antenna includes an anteriorly arranged main lobe and one or more posteriorly arranged sidelobes. At least one of the lobes of the first antenna is directed substantially at the non-terrestrial source and at least one of the nulls of the first antenna is directed substantially at the terrestrial source so that the first antenna receives the first signal from the non-terrestrial source, while substantially rejecting the second signal from the terrestrial source. At least one of the second antenna nodes is directed substantially at the terrestrial source so that the second antenna may receive the second signal. In one embodiment, to further enhance isolation between the first signal and the second signal, the second antenna is directed so that in addition to the foregoing, one of the second antenna nulls is directed substantially at the non-terrestrial source.
The present invention is also embodied in a method of transmitting information on at least one frequency simultaneously usable by at least one satellite transmitting to a satellite receive antenna having a sensitivity characterizable by a primary sensitive axis directed substantially at the satellite and a posterior secondary sensitive axis. The method is performed by transmitting the information from a terrestrially-based transmitter to a terrestrial receive antenna in a direction substantially away from the Equator. In another embodiment, the invention is embodied by a method for receiving information transmitted on at least one frequency simultaneously usable by at least one satellite transmitting to a satellite receive antenna having a sensitivity characterizable by a primary sensitive axis directed substantially at the satellite, and a posterior secondary sensitive axis. In this embodiment, the method is performed by receiving the information with a terrestrial receive antenna from a terrestrially-based transmitter transmitting the information in a direction substantially away from the Equator.